As conventional semiconductor memory devices, such as Flash memory and dynamic random access memory (DRAM), reach their scaling limits, research has focused on commercially viable low power, low operation voltage, high-speed, and high-density non-volatile memory devices. Materials having high dielectric constants (i.e., high static relative permittivity) are needed to provide sufficient capacitance in ever-smaller production-scale capacitor designs. Because of its high dielectric constant (or “k” value), titanium dioxide (TiO2) is being considered for use in non-volatile memory devices. TiO2 has three main crystalline phases: rutile, anatase, and brookite. The phase of a TiO2 crystal may depend on conditions of the TiO2 growth process, such as temperature and method of deposition. Of significance in semiconductor manufacture is that the dielectric constant of TiO2 varies based on properties such as crystalline phase, orientation, and deposition method. For example, TiO2 films grown on silicon substrates by atomic layer deposition (ALD) generally have an anatase crystalline structure, which has a dielectric constant of about 30. The anatase TiO2 may be converted to rutile TiO2 through an annealing process, including heating the TiO2 to a temperature above 800° C.
Rutile TiO2 may exhibit higher dielectric constants than anatase TiO2. For example, along the c-axis of the rutile TiO2, the dielectric constant may be about 170, while the dielectric constant along the a-axis may be about 90. Dielectric constants above about 55 are needed to meet capacitance requirements of DRAM in size ranges currently produced. As the scale of devices decreases, anatase TiO2 is not generally useful because its dielectric constant is too low.
A high deposition or anneal temperature is generally required to form rutile TiO2. For example, a semiconductor structure having anatase TiO2 thereon may be annealed by heating the anatase TiO2 to a temperature of about 800° C. During the anneal, the TiO2 crystalline structure may change from anatase to rutile. However, heating the anatase TiO2 to a temperature of about 800° C. may damage other structures on or within the semiconductor structure. For example, metal interconnects on or in the semiconductor structure may melt under such conditions. Rutile TiO2 may also be formed directly (i.e., without annealing anatase TiO2) by deposition at high temperature. Because of the processing temperatures needed to form rutile TiO2, use of rutile TiO2 may be limited to semiconductor structures that can tolerate high temperatures. DRAM and other structures may not withstand such temperatures. To take advantage of the high dielectric constants of rutile TiO2 on semiconductor structure that cannot tolerate high temperatures, it would be desirable to have a method of forming rutile TiO2 without using high temperatures required to deposit TiO2 in the rutile phase and without annealing anatase TiO2.
Japanese patent publication JP-A 2007-110111 describes a method of forming rutile TiO2 on a ruthenium electrode using a process temperature of less than 500° C. In that method, a ruthenium(IV) oxide pretreatment film is formed by exposing a ruthenium electrode to gaseous ozone (O3). Rutile TiO2 is then deposited in a film over the ruthenium(IV) oxide film, and a second electrode is formed over both films.